The proposed research program is a basic science (non-clinical) investigation of a potentially significant ultrasound (US)-induced biological effect, that is, whether the application of ultrasound contrast agents (UCAs) in humans adversely affects the vasculature. The medical profession benefits from these studies if it is shown that diagnostic US is a significant medical risk to the patient by advising clinicians about this risk, and by suggesting how the clinician can monitor the degree of risk. Likewise, the medical profession benefits from these studies if it is shown that diagnostic US is not a significant medical risk to the patient by eliminating vascular injury as a clinical concern. In either case, there is clear medical significance. The data necessary to decide this issue are not currently available. Today, we are faced with a significant challenge about the use of UCAs in humans, that is, the lack of knowledge as to whether the interaction of US with UCAs is a significant medical problem in humans. The FDA is also uncertain about UCAs' safety and/or effectiveness and is waiting until more is known about the risk of these agents before approving new UCAs, thus denying their well-known benefits to the patient. Studies have been designed to address directly this "safe-use" issue by an interdisciplinary group of investigators who have considerable experience with US-induced tissue damage studies, as well as considerable experimental and theoretical experience with US biophysics and related physics areas. Using our continued proactive approach, we propose to utilize our extensive US bioeffects/biophysics expertise, along with our pathological, nutritional, biostatistical modeling and animal model expertise, to determine whether the interaction of US with UCAs is a significant medical problem in humans. The experimental (animal-based) and theoretical (mechanism-development) research program has four specific aims, viz., 1) to determine the US thresholds for arterial damage in normal, healthy New Zealand white (NZW) rabbits when US interacts with UCAs in vivo; 2) to determine the US thresholds for arterial damage in post-exposure atherogenic diet-fed NZW rabbits when US interacts with UCAs in vivo; 3) to determine the US thresholds for arterial damage in pre-exposure atherogenic diet-fed NZW rabbits when US interacts with UCAs in vivo; and 4) to determine UCA thresholds in vitro. We are proposing 7 in vivo studies and a set of in vitro studies to accomplish our experimental and theoretical objectives.